1. Field
Embodiments of the present invention relate to aircraft nacelles and fan duct nozzles.
2. Related Art
An aircraft nacelle is a structure that surrounds an aircraft engine and broadly includes an inner cowl surrounding the engine and an outer cowl positioned outward of the inner cowl, forming an air duct or fan duct therebetween. Air flows in a forward to aft direction through the fan duct and out through an opening at an aft end of the nacelle, sometimes referred to as a nozzle.
In some applications, it is desired to vary the area of the nozzle. For example, by increasing the area of the nozzle during aircraft take-off, a fan within the nacelle is allowed to operate at a lower fan pressure ratio (FPR). This can increase the aircraft's engine efficiency, leading to reduced noise. Lowering the FPR enhances the fan stability and reduces its potential to stall. The nozzle area can be decreased as air temperature and density decrease at higher altitudes to gain added performance. At a cruising altitude, the nozzle area can be adjusted to compliment the engine cycle which has been optimized for cruise.
Moving and sliding components can be used to vary the nozzle area. Some prior art nacelles manipulate an outer flow surface of the fan duct (i.e., the outer cowl or translating sleeve of a thrust reverser) to vary the nozzle area. However, manipulating the outer flow surface can be complicated, particularly if the nacelle comprises a translating sleeve thrust reverser with its own actuators and moving parts.
Other prior art methods of varying nozzle area involve manipulating inner flow surfaces of the fan duct. Because it is generally desired to limit the size or volume of the nacelle at its aft end, prior art activation mechanisms for manipulating movable inner flow surfaces are usually positioned well forward of trailing edges of the air duct exit. Thus, the movable inner surfaces are designed with a large length to create the desired distance between the trailing edge and the activation mechanisms. However, this also increases the possibility of fan duct leakage at joints of the movable inner surface, negatively affecting the performance of the nozzle. Moreover, these movable inner surfaces can be heavy and require robust activation or manipulation components, which add even more undesired weight to the nacelle.
Some manipulated inner flow surfaces reduce the fan duct's area more at a location forward of the fan duct's exit than at the fan duct's exit, decreasing air flow efficiency. Furthermore, the location, size, and number of seams or joints of a variable nozzle can create undesired air leakage paths, causing premature wear on various components.
Accordingly, there is a need for an improved nacelle and variable fan duct nozzle that overcomes the limitations of the prior art.